The present invention relates to a water based negative photoresist composition that is particularly sensitive at wavelengths below 260 nanometers (deep ultraviolet), and a process for imaging such a photoresist.
Photoresist compositions are used in microlithographic processes for making miniaturized electronic components such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a thin film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to evaporate solvent in the photoresist composition and to fix the coating onto the substrate. The baked, coated surface of the substrate is next subjected to an image-wise exposure to imaging radiation.
This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are imaging radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate.
There are two types of photoresist compositions, negative-working and positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the photoresist composition exposed to the radiation become less soluble to a developer solution (e.g. a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to such a solution. Thus, treatment of an exposed negative-working photoresist with a developer causes removal of the non-exposed areas of the photoresist coating and the creation of a negative image in the coating, thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited. In a positive-working photoresist the developer removes the portions that are exposed. During the manufacture of devices it is sometimes desirable to use negative-acting photoresists to form images on a substrate.
The use of a negative-working, acid-sensitive photoresist composition is known in the prior art. Most of such prior art photoresist compositions use a crosslinking agent that reacts with the polymeric binder resin to form an insoluble film comprising a higher molecular weight polymer. Traditionally negative photoresists have been cast from organic solvents and developed using aqueous base developers. However, due to growing environmental concerns, negative photoresists that can be coated from a safe solvent, such as water or predominantly water, are highly desirable. Therefore, photoresists that are negative and water based are needed by the semiconductor industry. Furthermore, the trend toward miniaturization of integrated circuits has led to the development of photoresists that absorb at smaller and smaller wavelengths. Currently photoresists that can be imaged by wavelengths of light below 260 nanometers (nm) known as deep ultraviolet (DUV), are preferred. Thus, photoresists that are negative, water based and sensitive below 260 nm are even more desirable.
Aqueous based negative photoresists typically comprise a polymer, photoactive compound and crosslinking agent, where all the solid components are soluble in water or where water is the predominant solvent. Further, for photoresist that must be photoactive at wavelengths below 260 nm, the polymers and the crosslinking agent should be essentially nonabsorbing at the imaging wavelengths, this being particularly critical for the polymer, since it is the predominant proportion of the photoresist film. Polymers with minimal amounts of aromatic functionality, are desirable for imaging below 260 nm, and polymers with no aromatic functionality are especially desirable for imaging below 200 nm. Photoactive compounds that are essentially water-soluble and absorb at the exposure wavelength to generate an acid are required. Photoactive compounds that are based on derivatives of sulfonium salts that are water-soluble and undergo efficient photochemical transformation are particularly suitable. Similarly, crosslinkers that are essentially water-soluble and are very efficient crosslinkers in the presence of an acid are needed.
Examples of aqueous negative photoresists have been disclosed previously by Harvard et al (Macromolecules 1999, 32, 86-94 and Chem. Mater. 1999, 11, 719-725). Harvard et al (Chem. Mater. 1999, 11, 719-725) describes a photoresist composition based on polyvinylalcohol, (2,4-dihydroxyphenyl)dimethyl sulfonium triflate and hexamethoxymethymelamine in water.
Dimethylmultihydroxyphenylsulfonium salts are also disclosed in U.S. Pat. No. 5,648,196, but the polymers described therein contain aromatic functionality. Sakamizu et al (ACS Symposium Series, 614,124-36, 1995) evaluates photoresists containing aromatic polymers and water-soluble sulfonium salts. In Macromolecules 1999, 32,86-94, Harvard et al describes a two component water-soluble photoresist that uses (4-methoxyphenyl)dimethylsulfonium trifluoromethanesulfonate to transacetalize polymethacrylate containing sugar fuctionality.
U.S. Pat. No. 5,858,620 discloses a process for coating an imaged photoresist pattern with a nonphotosensitive aqueous solution of polyvinylacetal, or a mixture of polyvinylacetal and a crosslinking agent, and heating the solution to form a crosslinked coating over the photoresist pattern. The objective of this process is to reduce the dimensions of the open spaces on the substrate.
The present invention relates to a novel negative, aqueous photoresist composition comprising a polyvinylacetal polymer, a water-soluble photoactive compound and a crosslinking agent. The invention also relates to a process for forming a negative image from the novel photoresist composition.
The present invention relates to a novel negative aqueous photoresist composition, comprising a polymer comprising at least one unit with the structure (1), 
where,
R1 is hydrogen or (C1-C4) alkyl and n=1-4; a water-soluble photoactive compound; a crosslinking agent; and, a solvent composition.
The preferred photoactive compound has the structure (2), 
where,
R2 is independently hydrogen, alkyl, xe2x80x94O(alkyl), -(alkyl)OH, hydroxyphenyl or multihydroxyphenyl, R3 and R4 are independently (C1-C4) alkyl, m=1-3, and, Xxe2x88x92 is an anion. The solvent for the photoresist composition is preferably water or water and a (C1-C4) alkyl alcohol.
The invention further relates to a process for forming a negative image using the photoresist composition of this invention.